1. Field of the Invention
The present invention relates to a tension measuring apparatus and method for a piston ring mounted on a piston and inserted in the cylinder of an internal combustion engine or the like.
2. Description of the Prior Art
A piston ring must have a predetermined tension in order to apply a predetermined surface pressure to the inner circumferential surface of a cylinder when the piston ring is mounted on a piston and inserted in the cylinder of an internal combustion engine or the like. For this reason, the tension of the piston must be measured. For the measurement, a thin steel belt or the like is wound on the piston ring, and a tensile force is applied to the steel belt to reduce the diameter of the piston ring. When the diameter of the piston ring is reduced until the closed gap reaches a value at the cylinder insertion state, the applied tensile force is determined as the tension of the piston ring.
According to the first conventional case to measure a tension of a piston ring, an impact or a vibration is applied to the piston ring and the thin steel belt at the wound portion to remove the frictional force between them. The diameter of the piston ring is reduced until the closed gap reaches a value obtained in advance in a reference ring gauge. The tensile force applied at this time is measured by a load cell, a spring balancer, or the like.
According to the second conventional case to measure a tension of a piston ring (e.g., Japanese Utility Model Publication No. 2-667), a vibration is applied to the piston ring and the thin steel belt at the wound portion to remove the frictional force between them, and the length of the thin steel belt is adjustable. Once the length of the thin steel belt is set to obtain a predetermined closed gap, the predetermined closed gap is obtained automatically.
In any of the first and second conventional cases described above, since vibration or the like is applied to only the piston ring and the thin steel belt at the wound portion to remove the frictional force between them, the frictional force cannot be removed sufficiently. Therefore, the measured tension varies largely, and it is difficult to stably measure the tension of the piston ring at high precision.
In any of the first and second conventional cases described above, the closed gap of the piston ring must be measured at least once. However, it is not easy to obtain a closed gap close to that in the actual use state. In addition, measurement of the closed gap tends to cause an error and takes a comparatively long period of time. For these reasons, it is difficult to stably measure the tension of the piston ring at high precision within a short period of time.